Survey data reveal that more that 15 percent of otherwise healthy elderly report sleep disturbance. The most frequent complaints are: (1) inappropriate timing of sleep with profound sleepiness during the evening hours and very early morning awakening; and (2) poor consolidation of sleep and wakefulness, with long and frequent waking episodes after sleep onset and many voluntary or involuntary daytime naps. Thus far, the investigators have found that there are considerable age-related changes in the properties of the circadian system--which times sleep within the 24-hour day -- which may contribute to the disrupted sleep of the elderly. The investigators have also found that the human circadian pacemaker is more sensitive to light than previously recognized, and that properly timed exposure to bright light may be able to correct such age-related abnormalities in circadian functions. On the basis of our results, three testable hypotheses are proposed: (1) that healthy elderly subjects have a weaker (Type 1) circadian phase resetting response to the same light stimulus which induces strong (Type 0) circadian phase resetting in young subjects; (2) that elderly patients with idiopathic nocturnal sleep disruption (i.e., not secondary to identifiable sleep pathology) have significantly greater reductions of circadian amplitude and/or a more advanced circadian phase than those without a sleep disturbance; and (3) that phototherapy can reverse abnormalities of circadian organization in elderly patients with disrupted sleep and can lead to improved nocturnal sleep and daytime alertness. A series of experiments are proposed utilizing new methodologies that will allow direct measurement of the phase-resetting capacity of the endogenous circadian oscillator in the elderly across the full range of circadian phases. A second protocol is proposed to evaluate the correlation between specific sleep disturbances and abnormalities of the circadian timing system in the elderly, followed by an evaluation of the effectiveness of phototherapy in reversing any observed circadian abnormalities. This work has significant implications for gerontologic medicine. Sleep disorders in the elderly are an important cause of a diminished quality of life in otherwise healthy people. They frequently lead to an overconsumption of sleep medications with unfortunate physiological and psychological side effects. Greater understanding of the mechanisms underlying sleep disorders in the elderly as well as the development of non-invasive methods for their alleviation offer substantive promise for improved health of the elderly.